Casting Procedure, Particularly for an Engine Cylinder Head

ABSTRACT

Cast parts having internal cavities or passages are produced by using a main core and one or more secondary cores. The secondary cores are associated with the main core by coating the secondary cores with a material which dissolves on contact with mold metal. The cores to be coated can also be formed by hollow inserts made of a heat resistant material, filled with sand polymerized resin.

This application is a continuation of application Ser. No. 10/530,196 filed Apr. 4, 2005, now abandoned, which was the U.S. national stage of international application PCT/IT02/00771 filed Dec. 9, 2002, which claims priority from Italian Patent Application BS2002A000088, filed Oct. 4, 2002, all of which applications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates in general to the technology of metal casting. In particular, it relates to gravity chills and low-pressure processes that use cores to produce cavities in the casting. A typical example of such casting process is that used for making engine cylinder heads, where internal cores are necessary to produce the water jacket for the engine cooling water, the intake and exhaust ducts, and any other secondary cavity.

BACKGROUND OF THE INVENTION

Generally, for medium and large productions, engine cylinder heads are cast using a fixed outside mold, called a chill. Internal cores and sometimes external cores are required as well; these are inserted (assembled) into the chill to form a single body ready for casting.

When casting in a chill with sand and polymerized resin cores, the main difficulty is perfectly positioning the inside of the part to be cast, that is the cores, with the outside, so as to obtain the required dimensional accuracy. The cores are produced in corresponding molds, called core boxes, and then they are normally pre-assembled in the proximity of the chill.

The group of pre-assembled cores is moved by automatic devices (grippers and jigs) and laid (assembled) into the chill.

At this point, the molten metal, which fills the volume between the sand cores and the chill, is poured in.

Sand projections, called prints, are formed on the cores to keep the core group assembled in the desired position. These prints are laid into the and do not constitute part of the object resulting from the casting. In the specific case of cores for intake and exhaust ducts of an engine cylinder head, whose surfaces form the end shape of the casting, such cores are inserted into the water jacket core and during the step of moving the cores group to the chill, if the assembly is performed manually they are free due to the effect of the gaps that will be occupied by the metal thickness. Then, they laid by gravity into the lower zone of the corresponding passages provided into the water jacket core. The duct cores contact the drag (lower base) of the chill, in their final position.

When core assembly is performed with automatic systems, the ducts cores are held into suitable positions relative to the water jacket by a special automatic device, but normally only by the side of the flange coupling to the intake and exhaust manifolds.

The entire operation requires must be carried out carefully.

Since traditional technology provides for the water jacket core and the other cores to be molded separately, the water jacket core box interior must also be provided with the other parts resulting from the outside thickness of the cast—internal parts (ducts, etc.) and that are intended to house—during the subsequent core assembly—the other cores. However, since the ducts' outside parts do not undergo drafting as they normally are at half the height of the water jacket, mobile parts are currently used, controlled by gears, camshafts or, in the best case, by pneumatic cylinders, almost always moving on inclined axes.

For an engine cylinder head, in order to ease the extraction of these moving parts it is necessary to impart a greater inclination (draft angle) and deform the outside thickness of the ducts, but this requires excess material to obtain the minimum thickness required by the casting operation. This reduces the size of the water jacket core, making it more brittle and at the same time lower efficiency of the cooling circuit.

In other cases, the problem of passing the duct cores through the openings obtained in the water jacket cores is solved by dividing the latter horizontally into two halves, which are then attached to each other by an adhesive after the ducts have been inserted.

However, this results in higher production costs and lower quality of the finished product, particularly because of the casting flashes that may project into the water circulation compartment, and due to casting blowholes that may develop from the possible contact of molten metal with the half-core fixing adhesive.

Another casting process, called Lost Foam, involves making multiple polystyrene sectors using special dies. Once the sectors are attached to one another, they match the part to be cast. The polystyrene model thus obtained is coated and then put into a container, which is then filled by vibration with common sand or a similar material. Using a special pouring channel, made of polystyrene as well, the molten metal is poured into the container. As the polystyrene burns, it is replaced by the metal, so as to form the desired casting.

This process eliminates the need to make and lay polymerized sand cores. But is has the disadvantage—in the case of an engine cylinder head casting—that the duct shapes, even though coated, are not optimal since their surfaces are molded and therefore directly finished by the polystyrene surfaces. They may have junctions resulting from the coupling of polystyrene sectors. The necessary use of glue, actually, is the primary cause of blowholes. Thus, this process is seldom used.

SUMMARY OF THE INVENTION

An object of the present invention is to obviate the disadvantages of the prior art mentioned above, by providing a new casting procedure which allows one to obtain higher quality castings, thereby reducing the amount of scrap due to dimensional defects, and further introducing new design prospects.

Another object of the invention is to provide a casting procedure which allows a perfect relative positioning between each core and an easy insertion of the cores into the mold or into another core, whichever their shape.

Another object is to provide a casting procedure which allows considerable simplification of the core boxes, that is, without complex shapes, undercuts and connected moving parts, and which is therefore cheaper, more reliable and easier to achieve.

Another object of the invention is to provide a chill casting process for engine cylinder heads which allows one to make cores for intake and exhaust ducts without any deformation on the outside thickness and with the most varied and complex shapes. This may result in better engine performance and more advanced engines as regards exhaust gases.

Yet another object of the invention is to provide a casting procedure for engine cylinder heads which allows embedding inserts for the ducts into the casting, the inserts being made of a refractory material capable of standing the heat generated by the molten metal in order to obtain perfectly smooth ducts which should contribute to improving the engine efficiency.

These and other objects of the invention are achieved by a casting process as described below.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention will appear more clearly with reference to the attached indicative and non-limiting drawings. In the drawings:

FIG. 1 shows sand and polymerized resin cores for producing the intake and exhaust ducts of an engine cylinder head;

FIG. 2 shows a section view of the core boxes for molding the cores of FIG. 1, in a variation with inserts around the intake and exhaust ducts;

FIG. 3 shows the duct cores with inserts obtained with the core box of FIG. 2;

FIG. 4 shows a section of the duct cores inserted into the die for coating with foamed material;

FIG. 5 shows the group of valve seats and duct cores coated with foamed material;

FIG. 6 shows the water jacket core box still empty;

FIG. 7 shows the core box of FIG. 6 with the group of duct cores of FIG. 5 inserted therein;

FIG. 8 shows the group of cores, valve seats and foamed coating obtained by molding the water jacket core in the core box of FIG. 7, with the valve guides inserted into the coating;

FIG. 9 shows a complete chill core assembly scheme of the core group of FIG. 8;

FIGS. 9 a and 9 b show two enlarged details of the core assembly scheme, where black parts denote the difference in the shape and volume of the water jacket dimensions that can be obtained by the present invention as compared to the current art; and

FIG. 10 shows the core assembly scheme in the variation with inserts.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, the present invention relates to a casting procedure for obtaining castings provided with inside cavities. It is known to produce such cavities by laying into a mold one or more cores made or sand and polymerized resin or other material. Such cores, in turn, are inserted into special molds, called core boxes. Additional cores may be inserted separately, each into a respective core box, and then they are joined together before being laid (assembled) into the mold or chill. To this purpose, the cores are usually given complementary projections and cavities, called positive and negative prints, to support one another, and other sand projections intended to lay into suitable seats into the chill, which do not form part of the casting.

The procedure according to the present invention includes a step of coating one or more cores made of sand or other material with a layer of foamed material, such as polystyrene, only in the shaped zones, using a special die and then laying them into the chill.

The core coating material is intended to dissolve on contact with the casting metal, which replaces it thereby determining the required casting thickness, so that the finished casting surface will be determined by the quality of the core surface.

The procedure is especially advantageous in casting processes that use a main core and one or more secondary cores. According to the invention, after being placed in the usual way into respective core boxes, such secondary cores are laid into a die and coated with foamed material only in the shape zone, with the thickness required by the casting, and then they are laid (pre-assembled) into the main core box yet to be molded, that is, empty.

In order to receive the secondary cores that are already coated with foamed material, the main core box will be empty at the shapes of said secondary cores since shapes and thickness are replaced by the cores and by the coating layer. As a consequence, the main core box is much easier and cheaper to make since it allows eliminating any inside undercut and any moving parts required to contain the secondary cores. In addition the main core box only has the external prints of the secondary cores. Following the molding of the main core box with sand and polymerized resin, a single monolithic body is obtained, already assembled and having precise geometry. This body comprises the main core and the secondary cores, which are integrated with the main core by the coating that forms the casting thickness.

Such a monolithic body can then be easily carried and laid into the mold or chill.

The invention can be applied not only to cores of sand or of other material, but also to thin hollow inserts made of a refractory material, such as metal or composite material, and intended to be embedded into the casting for making the inside surfaces of the cavities perfectly smooth. From the dimensional point of view, the main core box is capable of receiving both sand cores or inserts coated with foamed material.

If inserts of metal or other material are to be embedded into the casting, and these inserts have an inside void corresponding to the core design, they must be laid into a specific core box which only considers the insert thickness in addition, and then it is molded. The resulting core will be provided with prints and embedded inserts, only in the shaped zone, and besides serving as support for the inserts, the core also prevents molten material from penetrating into the void part of the embedded inserts.

The property that allows the pre-assembly of already coated cores into a core box yet to be molded (void) allows for such a secondary cores to be made in geometrical shapes which would otherwise be not possible. Thus it is possible to obtain several passages, labyrinths and other shapes not previously possible, without requiring a successive assembly.

Consequently the described process results in the outside thickness of all secondary cores having no deformation and being perfectly shaped as in the design specification, which was not always possible with traditional technology, since often inside shapes of a main core box required mobile parts for drafting, which necessitated deformation.

This invention enables the casting designer to obtain castings which can even embed other adjacent parts that are currently cast separately, because of the constraints of the current traditional casting technology.

The inventive technology can also be used to stiffen fragile cores by pre-coating them with foamed material to facilitate handling or for a greater protection against breakage after pre-assembly into the mold, or for restricting the effect of metallostatic pressure.

The above advantages are obtained both by performing a pre-assembly directly into the die, and in this case the coating thickness may be equal to or smaller than the casting thickness, or by laying the pre-coated cores into another core box yet to be molded, and in this case the coating must be equal to the casting thickness.

In order to coat both cores and inserts with polystyrene or other equivalent material, it is necessary to have a specific die consisting of a single lower negative half and another upper negative half, whereby the positive shapes are form by the cores or inserts to be coated.

The mold is constructed with all core print seats being the same as the core boxes, molds or chills, considering the specific tolerances and thermal expansions.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The casting procedure described is especially but not exclusively adapted to be applied to a chill casting process of an engine cylinder head. As shown in FIG. 9, the main core 11 is the water jacket core that is that intended to create the coolant circulation passages, whereas the secondary cores mainly are those relating to the intake duct 12 and the exhaust duct 13.

The latter, plus any other secondary cores, such as for example those intended to create the exhaust gas circulation compartment and those that involve the water jacket core, are molded into respective core boxes in a traditional manner. Once molded, such cores are laid into a single mold 19 (FIG. 4) to be coated with foamed material 18, such as polystyrene. Valve seats 14,15 for the intake and exhaust valves, may be previously laid into the die, at special positions. Beforehand, the mold may be provided with mobile cylindrical pins 16, 17 intended to form seats 16′,17′ (FIG. 5) for the valve guides 16″, 17″.

Polystyrene 18, or equivalent material, is injected into the die so as to envelop the shaped zones of the cores inserted therein, with the required casting thickness, thereby excluding the core prints 12′, 13′. The outer diameters of the valve seats are embedded as well, aligned with the conical edges of the duct cores. For this specific application, the valve seats 14, 15 must have the proper machining stock on the inside diameter. The outside diameter of the valve seats is made with a taper equal to the inside one, such taper being required for the coating material to support and hold in position the valve seats during further handling, up to the assembly into the chill or die. The metal will then permanently lock the valve seats onto the casting.

The group consisting of ducts 12, 13 and of the valve seats 14, 15, all of which are lined with foamed material 18 and therefore comprising a single body (FIG. 5) is then laid (assembled) into the water jacket core box 20 (FIGS. 6,7). As mentioned above, the water jacket core box has a very simple structure since it is free from the shapes corresponding to the outside thickness of pre-assembled secondary cores.

In fact, in place of such shapes there are reference seats 20′ and negative prints 20″ (FIG. 6) intended to receive the secondary cores with the respective prints and the valve seats coated with foamed material. The core box therefore is free from any undercuts or mobile parts.

At this point the water jacket core box is filled with sand and polymerized resin, thereby producing a very accurate monolithic group in which the water jacket core 11 envelops and holds the outside thickness of the duct cores consisting of the foamed material on the shaped zones (FIG. 8). A perfect relative positioning between each core is therefore obtained as well.

When the entire group consisting of secondary cores, valve seats and foamed material has been molded into a single body with the water jacket core, the valve guides 17″ can be automatically inserted into the suitable seats obtained in the foamed material. Special sealing members are applied to the junctions between the foamed material and the upper half water jacket core box in order to prevent sand from infiltrating into the guide seats during molding.

The valve guides are solid (without central holes) since mechanical machining for inserting the valve stems is performed with valve guides embedded into the casting. Among the other things, this makes it possible to avoid the use of traditional stiffening bosses around the valve guide into the duct cores.

The valve guides have a negative circular groove at the portions embedded in the foamed material, to hold the valve guides in position in the casting metal when the latter replaces the foamed material.

In the upper portion of the valve guide there is often another core 21 for the oil gallery, as in FIG. 10, or a core for the tappet compartment, which has the risers (casting metal feeding during the shrinkage by cooling).

As a consequence, the upper end of the valve guides are always guided into suitable seats formed in the upper core or in the tappet compartment core, and therefore are locked into the correct position, even when the casting metal has dissolved or is dissolving the foamed material around the valve guides, without making the same valve guides collapse. At the bottom, the guides are inserted and locked in suitable seats 22 formed in the duct cores (FIG. 3).

At this point, the monolithic group comprising the water jacket core 11, the secondary cores, the valve seats, the valve guides, and the foamed material, along with other cores, such as core 21, can be laid into the chill (FIGS. 9,10). During the casting, the molten metal dissolves and replaces the foamed material, determining the required thickness and embedding the valve seats and the valve guides.

FIG. 10 shows the same assembly scheme described above, but here the duct cores consist of metal hollow inserts 23 (or are made of another material capable of standing the heat generated by the casting metal), fitted with sand and polymerized resin having a support function as well as serving to prevent any penetration of molten metal into the inserts. The interior of the inserts has the same dimensional features of the sand cores. Polymerized sand cores and inserts are molded into a specific core box must take into account the thickness of said inserts (FIG. 2).

At one end, the inserts butts against the valve seats whereas as the opposed end, they rest flush against the casting raw flange.

Since the intake and exhaust duct cores 12,13 and any other secondary cores are been coated with foamed material 18 in the water jacket core box 11, there is no design limit for the ducts or for other secondary cores. For example, the intake ducts may be connected to one another within a single chamber without any interruption in the horizontal direction to the upper parts of the valve seats. The chamber may even reach the intake manifold coupling flange and form a single chamber integral with the intake manifold, without creating any assembly problems with the water jacket. This concept may also be extended to inserts made of another material and embedded into the casting.

As a consequence, the head designer has a wide freedom of design since the current design constraints are eliminated, such as the required passage of the ducts through the water jacket. For example, as shown in FIGS. 9 a and 9 b, the water jacket compartment can have a more rounded design (black parts) in place of the current inclined surfaces and sharp edges to allow drafting. The outside duct core thickness is also free from deformations, with a constant and perfect thickness exactly as in the drawing specification.

In summary, the chill casting procedure proposed and applied to the production of an engine cylinder head yields the following advantages:

-   -   intake and exhaust ducts without any inside design constraints         and without deformations on the outside thickness, with         consequent constant casting thickness;     -   intake and exhaust ducts consisting of heat resistant inserts         embedded during casting;     -   higher geometrical accuracy in the position of the intake and         exhaust ducts and of the water jacket relative to the combustion         chambers;     -   water jacket with a greater water passage volume in the more         critical zones;     -   valve seats embedded during casting;     -   valve guides embedded during casting;     -   the possibility of eliminating the holes created by the prints         for supporting the water jacket core in the pouring step,         thereby eliminating the mechanical machining required to plug         such holes. 

1. A method for casting an engine cylinder head from molten metal, said method comprising steps of: forming a core from sand or other material, said core having a conical end towards a combustion chamber; providing a die for coating said core with a layer of a material suitable for dissolving upon contact with said molten metal; providing at least one valve seat having conical internal and external surfaces, wherein the tapering of the internal surfaces corresponds to the tapering of said conical end of the core; placing said valve seat in a respective reference provided in said die; placing said core in said die in such a way that the conical end of the core mates with said valve seat internal surface; and coating said core with said layer of material in such a way that said material envelops said valve seat around the external surface thereof.
 2. A method according to claim 1, further comprising a step of forming at least one seat for a valve guide, before said step of coating at least one core with a layer of a material adapted to dissolve in contact with the casting metal.
 3. A method according to claim 1, wherein said coating is applied to the core only in the zones and by the thickness of the casting shape.
 4. A method according to claim 3, wherein the coating is performed by injection molding on the core to be coated.
 5. A method according to claim 1, wherein a main core, which is the water jacket core intended to form the coolant circulation passages, and secondary cores, which mainly are ducts cores for the intake and exhaust ducts, are provided.
 6. A method according to claim 5, wherein the main core is formed into a special core box, along with one or more secondary cores intended to be associated to said main core, and further comprising steps of: coating at least one secondary core, only in the zones and by the shaped thickness, with a layer of material intended to dissolve in contact with the molten metal; inserting the group consisting of at least one secondary core and of at least one valve seat, all coated with said layer of material intended to dissolve in contact with the molten metal into the main core box yet to be molded; molding the main core box; and inserting the monolithic group comprising the water jacket core, the secondary cores, the at least one valve seat, the layer of material intended to dissolve in contact with the molten metal that keeps them firmly connected, into the mold or chill intended to receive the molten metal.
 7. A method of an engine cylinder head according to claim 6, wherein the secondary cores include cores of the intake and exhaust ducts, wherein said duct cores are laid into a single die to be coated with the coating material layer so as to form a single body to be laid into the special water jacket core box.
 8. A method according to claim 7, wherein valve seats for the intake and exhaust ducts are first laid into the die, the injected material enveloping said valve seats on the outer diameter.
 9. A method according to claim 1, wherein prior to assembly monolithic group into the mold or chill, at least one valve guide for the intake and/or exhaust valve is inserted into the monolithic group comprising the main core and the secondary coated cores.
 10. A method according to claim 1, wherein the cores to be coated comprise hollow inserts made of a heat resistant material, wherein the cavity represents the shape according to the drawing.
 11. A method according to claim 1, wherein the cores to be coated comprise a hollow insert for the shaped zones only, made of a heat resistant material filled with sand and polymerized resin to form prints and prevent metal infiltrations.
 12. A method according to claim 1, wherein the coating material of the sand and polymerized resin cores or of the inserts is a foamed material.
 13. A method according to claim 12, wherein the foamed material is polystyrene.
 14. A main core box for the method according to claim 5, said box consisting of only two portions intended to be closed onto one another, and being free from undercuts and moving parts, and having seats and negative prints for receiving and blocking into position the secondary cores pre-coated with the coating material.
 15. A die for coating the intake and exhaust duct cores in an engine cylinder head casting process according to claim 5, said die comprising cylindrical mobile pins intended to form at least one seat into the coating material for the valve guides of the intake and/or exhaust valves.
 16. A die for coating the intake and exhaust duct cores in an engine cylinder head casting process according to claim 5, wherein the die is configured to receive at least one valve seat for the intake and/or exhaust valves.
 17. An engine cylinder head having embedded therein at least one embedded hollow insert made of metal or other heat resistant material, whose interior forms the design of the corresponding intake and exhaust duct.
 18. An engine cylinder head casting, having at least one embedded valve seat or valve guide.
 19. A method of casting parts having internal cavities or holes, said method comprising steps of laying into a mold or chill intended to receive the molten metal one or more cores made of sand or other material, wherein each core is formed separately into a proper core box, and, before laying into the mold or chill, coating at least one core with a layer of a material adapted to dissolve in contact with the casting metal, and further comprising, before said coating step, forming at least one seat for a valve guide.
 20. A method for casting an engine cylinder head, wherein said cylinder head has at least one water jacket for the engine coolant circulation at least an intake and/or exhaust duct, and at least one valve seat for the intake and/or exhaust valve, said method comprising steps of laying into a mold or chill intended to receive the molten metal one or more cores made of sand or other material and provided separately into proper core boxes, at least the core of the duct being coated with a layer of a material adapted to dissolve in contact with the casting metal before being laid into the mold or chill, and embedding said at least one valve seat guide into said layer of a material adapted to dissolve in contact with the casting metal, in such a way that during the casting, the molten metal will dissolve and replace the layer of a material adapted to dissolve in contact with the casting metal, determining the required thickness and embedding said valve seat or guide.
 21. Apparatus for casting an engine cylinder head, wherein said cylinder head presents at least a water jacket for the engine coolant circulation, at least an intake and/or exhaust duct, at least one valve guide for the intake and/or exhaust valve, wherein said water jacket and duct are obtained by laying into a mold or chill intended to receive the molten metal one or more cores made of sand or other material and formed separately into proper core boxes, and wherein at least the core of the duct is coated with a layer of a material adapted to dissolve in contact with the casting metal before being laid into the mold or chill, and further comprising embedding said at least one valve guide into said layer of material in such a way that during the casting, the molten metal will dissolve and replace said layer of material, determining the required thickness and embedding said valve guide.
 22. Apparatus for casting an engine cylinder head, wherein said cylinder head presents at least an intake and/or exhaust duct, at least one valve seat or valve guide for the intake and/or exhaust valve, wherein said duct is obtained with one or more cores made of sand or other material, and wherein at least the core of the duct is coated with a layer of a material adapted to dissolve in contact with the casting metal, and further comprising means for embedding said at least one valve seat or guide into said layer of a material adapted to dissolve in contact with the casting metal, in such a way that during the casting, the molten metal will dissolve and replace the layer of a material adapted to dissolve in contact with the casting metal, determining the required thickness and embedding said valve seat or guide. 